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| United States Patent |
5,321,194
|
|
Apelian
, et al.
|
June 14, 1994
|
N-olefin skeletal isomerization process using dicarboxylic acid treated
zeolites
Abstract
A method for skeletal isomerization of linear olefins to iso-olefins, e.g.,
n-butenes to isobutylene, over a catalyst comprising medium pore zeolite,
e.g., a zeolite selected from ZSM-22, ZSM-23, and ZSM-35. Treatment of the
zeolite with dicarboxylic acid, e.g., oxalic acid, significantly reduces
aging rate and increases cycle length of the catalyst.
| Inventors:
|
Apelian; Minas R. (Vincetown, NJ);
Rahmim; Iraj (Turnersville, NJ);
Fung; Anthony S. (Chadds Ford, PA);
Huss, Jr.; Albin (Chadds Ford, PA)
|
| Assignee:
|
Mobil Oil Corporation (Fairfax, VA)
|
| Appl. No.:
|
881278 |
| Filed:
|
May 11, 1992 |
| Current U.S. Class: |
585/671 |
| Intern'l Class: |
C07C 005/27 |
| Field of Search: |
585/671
|
References Cited
U.S. Patent Documents
| 3442795 | May., 1969 | Kerr et al. | 208/120.
|
| 3992466 | Nov., 1976 | Plank et al. | 260/671.
|
| 4388177 | Jun., 1983 | Bowes et al. | 208/111.
|
| 4886925 | Dec., 1989 | Harandi | 585/331.
|
| 4922048 | May., 1990 | Harandi | 585/310.
|
| 4996386 | Feb., 1991 | Hamilton, Jr. et al. | 585/646.
|
| 5057635 | Oct., 1991 | Gajda | 585/259.
|
| Foreign Patent Documents |
| 0026041 | Apr., 1981 | EP.
| |
| 0135261 | Apr., 1985 | EP.
| |
| 0247802 | Dec., 1987 | EP.
| |
| 0259526 | Mar., 1988 | EP.
| |
| 3246495 | Jun., 1984 | DE.
| |
Other References
Chemical Abstracts: vol. 85, No. 85: 194867m, 1976.
|
Primary Examiner: Pal; Asok
Assistant Examiner: Achutamurthy; P.
Attorney, Agent or Firm: McKillop; Alexander J., Santini; Dennis P., Hobbes; Laurence P.
Claims
It is claimed:
1. A method for conversion of linear olefins to corresponding iso-olefins
of the same carbon number which comprises contacting a linear
olefin-containing organic feedstock with a catalyst comprising a zeolite
sorbing 10-40 mg 3 methylpentane at 90.degree. C., 90 torr, per g dry
zeolite in the hydrogen form, under skeletal isomerization conditions,
said zeolite having been contacted with dicarboxylic acid under conditions
sufficient to effect a significant reduction in surface acidity of the
zeolite as determined by tri-tertiarybutylbenzene conversion without
substantially reducing the overall activity of the zeolite as indicated by
alpha value.
2. The method of claim 1 wherein said zeolite is selected from the group
consisting of those having the framework structure of ZSM-22, ZSM-23, and
ZSM-35, said dicarboxylic acid is oxalic acid, and said conversion is
carried out at temperatures between about 100.degree. and 750.degree. C.,
weight hourly space velocities based on linear olefins in said feedstock
between 0.1 and 500 WHSV, and linear olefin partial pressures between 2
and 2000 kPa.
3. The method of claim 1 wherein said conversion is carried out at
temperatures between about 200.degree. and 600.degree. C., weight hourly
space velocities based on linear olefin in said feedstock between 1 and
400 WHSV; and linear olefin partial pressures between 10 and 500 kPa.
4. The method of claim 1 wherein said zeolite is ZSM-22.
5. The method of claim 1 wherein said zeolite is ZSM-23.
6. The method of claim 1 wherein said zeolite is ZSM-35.
7. The method of claim 1 wherein said conversion is at least 10 wt % and
has a linear olefin to iso-olefin selectivity of at least 80 wt %.
8. The method of claim 1 wherein said feedstock comprises C.sub.4 to
C.sub.10 linear olefins.
9. The method of claim 1 wherein said feedstock comprises C.sub.4 to
C.sub.6 linear olefins.
10. The method of claim 1 wherein said catalyst comprises 10 to 99 wt % of
a refractory inorganic oxide binder.
11. The method of claim 1 wherein said catalyst comprises 20 to 70 wt % of
a silica binder.
12. The method of claim 1 wherein said surface acidity is reduced by at
least 20%.
13. The method of claim 1 wherein said surface acidity is reduced by at
least 50%.
14. The method of claim 1 wherein said contacting results in less than
about 10% loss of crystallinity.
15. The method of claim 1 wherein said dicarboxylic acid is an aqueous
dicarboxylic acid solution.
16. The method of claim 1 wherein said dicarboxylic acid is in a
concentration in the range of from about 0.01 to about 4M.
17. The method of claim 1 wherein said dicarboxylic acid is selected from
the group consisting of oxalic, malonic, succinic, glutaric, adipic,
maleic, phthalic, isophthalic, terephthalic, fumaric, tartaric and
mixtures thereof.
18. The method of claim 1 wherein said dicarboxylic acid is oxalic acid.
19. The method of claim 1 wherein said contacting is for a time of at least
about 10 minutes, at a temperature in the range of from 15.degree. C. to
93.degree. C. (60.degree. F. to 200.degree. F.).
20. A method for conversion of linear olefins to corresponding iso-olefins
of the same carbon number which comprises contacting a linear
olefin-containing organic feedstock with a catalyst comprising a zeolite
having a Pore Size Index of 20 to 26, under skeletal isomerization
conditions, said zeolite being contacted with dicarboxylic acid under
conditions sufficient to effect a significant reduction in surface acidity
of the zeolite as determined by tri-tertiarybutylbenzene conversion
without substantially reducing the overall activity of the zeolite as
indicated by alpha value.
Description
Related Applications
This application is related by subject matter U.S. patent application Ser.
No. 07/881,282, filed herewith, now U.S. Pat. No. 5,242,676 and Serial No.
07/760,287, filed Sept. 16, 1991, now abandoned.
FIELD OF THE INVENTION
This invention relates to a method for the skeletal isomerization of
n-olefin-containing, e.g., n-butene-containing, hydrocarbon streams to
iso-olefin-rich, e.g., isobutene-rich product streams. The process uses a
zeolitic catalyst composition treated with dicarboxylic acid to
selectively dealuminate the crystal surface of the zeolite component. The
catalyst composition employed exhibits improved stability and extended
cycle length.
BACKGROUND OF THE INVENTION
The demand for iso-alkenes has recently increased in response to a greater
demand for oxygenated gasoline additives. For example, relatively large
amounts of isobutene are required for reaction with methanol or ethanol
over an acidic catalyst to produce methyl tert-butyl ether (MTBE) or ethyl
tert-butyl ether (ETBE) which is useful as an octane enhancer for unleaded
gasolines. Isoamylenes are required for reaction with methanol over an
acidic catalyst to produce tert-amyl methyl ether (TAME). With passage of
the Clean Air Act in the United States mandating increased gasoline
oxygenate content, MTBE, ETBE and TAME have taken on new value as a
clean-air additive, even for lower octane gasolines. Lead phasedown of
gasolines in Western Europe has further increased the demand for such
oxygenates.
An article by J. D. Chase, et al., Oil and Gas Journal, Apr. 9, 1979,
discusses the advantages one can achieve by using such materials to
enhance gasoline octane. The blending octane values of MTBE when added to
a typical unleaded gasoline base fuel are RON=118, MON=101, R+M / 2=109.
The blending octane values of TAME when added to a typical unleaded
gasoline base fuel are RON=112, MON=99, R+M / 2=106. Isobutene (or
isobutylene) is in particularly high demand as it is reacted with methanol
to produce MTBE.
The addition of shape-selective zeolite additives such as ZSM-5 to cracking
catalysts, e.g., those used in fluidized catalytic cracking (FCC), is
beneficial in producing gasoline boiling range product of increased octane
rating. However, increased amounts of olefins result, including n-butenes,
creating a need for their conversion to higher value products such as
isobutene which can be used to produce MTBE.
Butene exists in four isomers: butene-1, cis-butene-2, its stereo-isomer
trans-butene-2, and isobutene. Conversions between the butenes-2 is known
as geometric isomerization, whereas that between butene-1 and the
butenes-2 is known as position isomerization, double-bond migration, or
hydrogen-shift isomerization. The aforementioned three isomers are not
branched and are known collectively as normal or n-butenes. Conversion of
the n-butenes to isobutene, which is a branched isomer, is widely known as
skeletal isomerization.
The reaction of tertiary olefins with alkanol to produce alkyl tertiary
alkyl ether is selective with respect to iso-olefins. Linear olefins are
unreactive in the acid catalyzed reaction, even to the extent that it is
known that the process can be utilized as a method to separate linear and
iso-olefins. The typical feedstream of FCC C.sub.4 or C.sub.4 + crackate
used to produce tertiary alkyl ethers in the prior art which contains
normal butene and isobutene utilizes only the branched olefin in
etherification. This situation presents an exigent challenge to workers in
the field to discover a technically and economically practical means to
utilize linear olefins, particularly normal butene, in the manufacture of
tertiary alkyl ethers.
In recent years, a major development within the petroleum industry has been
the discovery of the special catalytic capabilities of a family of zeolite
catalysts based upon medium pore size shape selective metallosilicates.
Discoveries have been made leading to a series of analogous processes
drawn from the catalytic capability of zeolites in the restructuring of
olefins.
European Patent 0026041 to Garwood, incorporated herein by reference,
discloses a process for the restructuring of olefins in contact with
zeolite catalyst to produce iso-olefins, followed by the conversion of
iso-olefins to MTBE and TAME. The restructuring conditions comprise
temperatures between 204.degree. C. and 315.degree. C. and pressure below
51 kPa.
In European Patent 0247802 to Barri et al., it is taught that linear
olefins can be restructured in contact with zeolite catalyst, including
Theta-1 (ZSM-22) and ZSM-23, to produce branched olefins. The
restructuring conditions comprise temperature between
200.degree.-550.degree. C., pressure between 100 and 5000 kPa and WHSV
between 1 and 100. Selectivities to isobutene up to 91.2% are reported
using a calcined Theta-1 tectometallosilicate at 400.degree. C. and 30.6%
1-butene conversion.
U.S. Pat. No. 3,992,466 to Plank et al. teaches the use of ZSM-35 as a
catalyst for hydrocarbon conversion reactions, including "isomerization of
aromatics, paraffins and olefins."
U.S. Pat. No. 4,922,048 to Harandi discloses the use of a wide variety of
medium pore size zeolites, e.g., ZSM-5, ZSM-11, ZSM-12, ZSM-22, ZSM-23,
ZSM-35 and ZSM-48, in low temperature (232.degree.-385.degree. C.) olefin
interconversion of C.sub.2 -C.sub.6 olefins to products including tertiary
C.sub.4 -C.sub.5 olefins and olefinic gasoline.
U.S. Pat. No. 4,886,925 to Harandi discloses low pressure high temperature
conversion of light olefins to produce higher olefins rich in isoalkenes.
The process converts C.sub.2+ n-alkenes to a product comprising C.sub.4
-C.sub.6 alkenes rich in iso-alkenes, C.sub.7+ olefinic gasoline boiling
range hydrocarbons, and unconverted hydrocarbons over ZSM-5. The reference
teaches further treatment of the alkene effluent with methanol in the
presence of medium pore size zeolites such as ZSM-5, ZSM-11, ZSM-12,
ZSM-35, ZSM-38 and ZSM-48.
U.S. Pat. No. 4,996,386 to Hamilton, Jr. discloses concurrent isomerization
and disproportionation of hydrocarbon olefins using a
ferrierite/Mo/W/Al.sub.2 O.sub.3 catalyst. The catalyst exemplified
produces fewer branched olefins than a comparable material free of
ferrierite and the reference teaches that ferrierite-containing catalysts
exhibit improved selectivity to linear olefins than conventionally
prepared disproportionation catalysts.
All of the above references are incorporated herein by reference.
Despite the efforts exemplified in the above references, the skeletal
isomerization of olefins, e.g., to produce isobutene, has been hampered by
relatively low selectivity to isobutene perhaps owing to the reactivity of
these olefins. It is further known that skeletal isomerization becomes
more difficult as hydrocarbons of lower molecular weight are used,
requiring more severe operating conditions, e.g., higher temperatures and
lower linear olefin partial pressures.
Generally, the conversion of n-butenes to iso-butene is conducted at
selectivities below 90%. In order to obtain higher selectivities,
operation at high temperatures (>500.degree. C.) and with high nitrogen
feed dilution (butene partial pressure, typically less than 5 psia (34.5
kPa)) is generally required. Selectivities of greater than 90%, 95% or
even 99% are highly advantageous in commercial conversion of n-butenes to
isobutene in order to avoid the need to separate out materials other than
n-butene from the product stream. Such high selectivities will permit
direct introduction (cascading) or indirect introduction of the isomerizer
effluent to an etherification zone where isobutene is reacted with alkanol
to produce alkyl tert-butyl ether, e.g., MTBE. Unconverted n-butenes in
the isomerizer effluent can be withdrawn either before the etherification
zone or preferably, from the etherification zone effluent insofar as the
etherification reaction utilizes only the isobutene component of the
isomerizer stream. Unreacted n-butenes from the etherification zone
effluent can be recycled to the isomerizer where they are converted to
isobutene at high selectivity. If the recycle stream contains not only
unconverted linear olefins, e.g., n-butenes, but also by-product such as
other olefins (e.g., propylene) or paraffins, they have to be removed from
the recycle stream, such as by distillation or by taking a slip stream.
These removal steps are expensive and can lead to considerable loss of not
only the by-products but butenes as well. These losses are larger when the
by-products formed are present in higher concentration. Thus, even small
improvements in the isobutene selectivity during n-butene isomerization
have a major effect on the commercial viability of the process. However,
high selectivities in skeletal isomerization processes have generally
required low linear olefin partial pressures and high temperatures which
place substantial limitations on such processes. It would, therefore, be
advantageous to provide a skeletal isomerization catalyst capable of
maintaining relatively high selectivity at low temperatures and high
linear olefin partial pressures.
Certain medium pore size zeolites have been shown to be highly effective in
skeletal isomerization of normal olefins. Such zeolites include those
selected from the group consisting of zeolites having the framework
structure of ZSM-22, ZSM-23, and ZSM-35.
Although such catalyst compositions exhibit adequate thermal stability and
cycle length, improving such properties would be economically advantageous
by extending catalyst life and reactor down time.
SUMMARY OF THE INVENTION
The present invention provides a method for highly selective conversion of
linear olefins to corresponding iso-olefins of the same carbon number,
e.g., n-butenes to isobutene, which comprises contacting, under skeletal
isomerization conditions, a linear olefin-containing organic feedstock
with a catalyst comprising a zeolite having a constraint index of 1 to 12,
preferably one sorbing in its intracrystalline voids 10 mg to 40 mg
3-methylpentane at 90.degree. C., 90 torr, per gram dry zeolite in the
hydrogen form, said zeolite having been treated by contact with a
dicarboxylic acid. The high selectivity of the treated catalysts employed
in the present invention results in large part from isomerization
occurring without significant conversion to lighter and heavier molecules.
This phenomenon, it is believed, is a consequence of the pore structure of
the zeolite component of the catalyst which promotes somerization at a
much faster rate than the reaction by which say, butene, is converted to
lighter (mostly propylene) and heavier olefins (olefin interconversion
reactions). Moreover, such isomerization takes place without significant
cracking of the feed or hydrogenation or dehydrogenation effects resulting
in the formation of, say, n-butane or butadiene. The present invention is
particularly useful in that a dicarboxylic acid treatment of the zeolite
can selectively deactivate the surface acidity of the zeolite without
substantially reducing the alpha value (as hereinafter described) or
overall activity of the zeolite, resulting in reduced catalyst aging and
longer cycle length.
DESCRIPTION OF THE FIGURE
The Figure depicts catalyst aging for oxalic acid treated and untreated
silica bound ZSM-35.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides a process which converts a linear
olefin-containing hydrocarbon feedstream to an iso-olefin rich product at
high iso-olefin selectivity over a dicarboxylic acid-treated zeolite
catalyst under skeletal isomerization conditions.
The skeletal isomerization reaction of the present invention is carried out
at temperatures between 100.degree. and 750.degree. C.; weight hourly
space velocity based on linear olefin in the feed between 0.1 and 500
WHSV; and linear olefin partial pressure between 2 and 2000 kPa. The
preferred conditions are temperatures between 200.degree. and 600.degree.
C., more preferably between 250.degree. and 550.degree. C., WHSV between 1
and 400, more preferably between 5 and 100; and a linear olefin partial
pressure between 10 and 500 kPa, more preferably between 20 and 200 kPa.
Under these conditions the conversion of linear olefin, e.g., n-butene,
can be at least 10%, preferably at least 25% and more preferably at least
35%. Generally, economically feasible isomerization is maintained at
n-olefin conversion levels above 25 wt %, preferably above 30 wt %.
The present invention is especially suited to processes carried out at high
linear olefin to iso-olefin selectivity, e.g, at least 75% at relatively
low conversion temperatures and high linear olefin partial pressures. Such
processes can maintain selectivities of at least 90, 92 or 95% at a
conversion temperature less than or equal to 550.degree., 400.degree. or
even 350.degree. C., and linear olefin partial pressures above 2 psia (14
kPa), e.g above 5 psia (34 kPa). Zeolites used in these processes treated
with dicarboxylic acid exhibit maintain substantially equivalent overall
catalytic activity as measured by alpha value. However, the treated
zeolites retain their operable activity (permitting overall n-olefin
conversion of at least 30%) for a significantly longer period,
representing a reduction in aging rate of at least 25%, preferably at
least 40% or even 50%.
Preferred feedstreams include C.sub.4 or C.sub.4 + hydrocarbon feedstreams.
Linear olefins suited to use in the present invention may be derived from
a fresh feedstream, preferably comprising n-butenes and/or n-pentenes, or
from the effluent of an iso-olefin etherification reactor which employs
alkanol and C.sub.4 or C.sub.4 + hydrocarbon feedstock. Typical
hydrocarbon feedstock materials for isomerization reactions according to
the present invention include olefinic streams, such as cracking process
light gas containing butene isomers in mixture with substantial amounts of
paraffins including n-butane and isobutane. The C.sub.4 components usually
contain a major amount of unsaturated compounds, such as 10-40% isobutene,
20-55% linear butenes, and small amounts of butadiene. Also, C.sub.4 +
heavier olefinic hydrocarbon streams may be used, e.g., C.sub.4 to
C.sub.10, preferably C.sub.4 to C.sub.6 olefinic hydrocarbon streams.
Catalyst
Medium pore size zeolites useful in this invention comprise intermediate
pore size zeolites having a silica to alumina ratio of at least about 12
and a Constraint Index of about 1 to 12. The Constraint Index relates to
zeolite pore size, and will be more fully described below. Examples of
such zeolites are members of a novel class of zeolites that exhibit
unusual properties and include ZSM-5, ZSM-11, ZSM-12, ZSM-22, ZSM-23,
ZSM-35, and ZSM-48. Although these zeolites have unusually low alumina
contents, i.e., high silica to alumina ratios, they are active for
converting organic compounds. The activity is surprising since catalytic
activity is generally attributed to framework aluminum atoms and/or
cations associated with these aluminum atoms. These zeolites have an
intracrystalline sorption capacity for normal hexane which is greater than
that for water, i.e., they exhibit "hydrophobic" properties. An important
characteristic of the crystal structure of this class of zeolites is that
it provides constrained access to and egress from the intracrystalline
free space by virtue of having an effective pore size intermediate between
the small pore Linde A and the large pore Linde X, i.e., the pore windows
of the structure have about a size such as would be provided by
10-membered rings of oxygen atoms. It is to be understood, of course, that
these rings are those formed by the regular disposition of the tetrahedra
making up the anionic framework of the crystalline aluminosilioate, the
oxygen atoms themselves being bonded to the silicon or aluminum atoms at
the centers of the tetrahedra. The silica to alumina ratio referred to may
be determined by conventional analysis. This ratio is meant to represent,
as closely as possible, the ratio in the rigid anionic framework of the
zeolite crystal and to exclude aluminum in the binder or in cationic or
other form within the channels.
The medium pore size zeolites referred to herein have an effective pore
size such as to freely sorb normal hexane. In addition, the structure must
provide constrained access to larger molecules. It is sometimes possible
to judge from a known crystal structure whether such constrained access
exists. For example, if the only pore windows in a crystal are formed by
8-membered rings of oxygen atoms, then access to molecules of larger
cross-section than normal hexane is excluded and the zeolite is not of the
medium pore size type. Windows of 10-membered rings are preferred,
although in some instances excessive puckering of the rings or pore
blockage may render these zeolites ineffective.
Rather than attempt to judge from crystal structure whether or not a
zeolite possesses the necessary constrained access to molecules larger
than normal paraffins, a simple determination of the "Constraint Index",
or C.I., as herein defined may be made by passing continuously a mixture
of an equal weight of normal hexane and 3-methylpentane over a small
sample, approximately one gram or less, of zeolite at atmospheric pressure
according to the following procedure. A sample of the zeolite, in the form
of pellets or extrudate, is crushed to a particle size about that of
coarse sand and mounted in a glass tube. Prior to testing, the zeolite is
treated with a stream of air at 1000.degree. F. for at least 15 minutes.
The zeolite is then flushed with helium and the temperature is adjusted
between 550.degree. F. and 950.degree. F. to give an overall conversion
between 10% and 60%. The mixture of hydrocarbons is passed at 1 liquid
hourly space velocity (i.e., 1 volume of liquid hydrocarbon per volume of
zeolite per hour) over the zeolite with a helium dilution to give a helium
to total hydrocarbon mole ratio of 4:1. After 20 minutes on stream, a
sample of the effluent is taken and analyzed, most conveniently by gas
chromatography, to determine the fraction remaining unchanged for each of
the two hydrocarbons.
The C.I. is calculated as follows:
##EQU1##
The Constraint Index approximates the ratio of the cracking rate constants
for the two hydrocarbons. Zeolites suitable for the present invention are
those having a Constraint Index of 1 to 12. C.I. values for some typical
zeolites are:
TABLE I
______________________________________
CAS C. I.
ZSM-4 0.5
ZSM-5 8.3
ZSM-11 8.7
ZSM-12 2
ZSM-23 9.1
ZSM-35 4.5
ZSM-38 2
TMA Offretite 3.7
Beta 0.6-2
H-Zeolon (mordenite)
0.4
REY 0.4
Amorphous Silica-Alumina
0.6
Erionite 38
______________________________________
The above-described Constraint Index is an important definition of zeolites
which are useful in the instant invention. The very nature of this
parameter and the recited technique by which it is determined, however,
admit of the possibility that a given zeolite can be tested under somewhat
different conditions and thereby have different Constraint Indices.
Constraint Index seems to vary somewhat with severity of operation
(conversion) and the presence or absence of binders. Therefore, it will be
appreciated that it may be possible to so select test conditions to
establish more than one value in the range of 1 to 12 for the Constraint
Index of a particular zeolite. Such a zeolite exhibits the constrained
access as herein defined and is to be regarded as having a Constraint
Index of 1 to 12. Also contemplated herein as having a Constraint Index of
1 to 12 and therefore within the scope of the novel class of highly
siliceous zeolites are those zeolites which, when tested under two or more
sets of conditions within the above-specified ranges of temperature and
conversion, produce a value of the Constraint Index slightly less than 1,
e.g., 0.9, or somewhat greater than 12, e.g., 14 or 15, with at least one
other value of 1 to 12. Thus, it should be understood that the Constraint
Index value as used herein is an inclusive rather than an exclusive value.
That is, a zeolite when tested by any combination of conditions within the
testing definition set forth hereinabove and found to have a Constraint
Index of 1 to 12 is intended to be included in the instant catalyst
definition regardless that the same identical zeolite tested under other
defined conditions may give a Constraint Index value outside of 1 to 12.
For medium pore size zeolites of very high silica to alumina ratio, such as
1600:1, the Constraint Index cannot be measured reliably because of the
low activity of the zeolite. In such cases reliance on X-ray pattern is
useful.
The class of zeolites defined herein is exemplified by ZSM-5, ZSM-11,
ZSM-12, ZSM-22, ZSM-23, ZSM-35, ZSM-38, ZSM-48, and other similar
materials.
U.S. Pat. No. 3,702,886 describing and claiming ZSM-5 is incorporated
herein by reference.
ZSM-11 is more particularly described in U.S. Pat. No. 3,709,979, the
entire content of which is incorporated herein by reference.
ZSM-12 is more particularly described in U.S. Pat. No. 3,832,449, the
entire content of which is incorporated herein by reference.
ZSM-38 is more particularly described in U.S. Pat. No. 4,046,859, the
entire content of which is incorporated herein by reference.
ZSM-48 is more particularly described in U.S. Pat. No. 4,397,827, the
entire content of which is incorporated herein by reference.
The specific zeolites described, when prepared in the presence of organic
cations, are substantially catalytically inactive, possibly because the
intra-crystalline free space is occupied by organic cations from the
forming solution. These cations are removed by heating in an inert
atmosphere at 1000.degree. F. for one hour, for example, followed by base
exchange with ammonium salts followed by calcination at 1000.degree. F. in
air.
The zeolites referred to above have a crystal framework density, in the dry
hydrogen form, of not less than about 1.6 grams per cubic centimeter. The
dry density for known crystal structures may be calculated from the number
of silicon plus aluminum atoms per 1000 cubic Angstroms, as given, e.g.,
on Page 19 of the article on Zeolite Structure by W. M. Meier. This paper,
the entire contents of which are incorporated herein by reference, is
included in "Proceedings of the Conference on Molecular Sieves, London,
April 1967," published by the Society of Chemical Industry, London, 1968.
When the crystal structure is unknown, the crystal framework density may
be determined by classical pycnometer techniques. For example, it may be
determined by immersing the dry hydrogen form of the zeolite in an organic
solvent not sorbed by the crystal. Or, the crystal density may be
determined by mercury porosimetry, since mercury will fill th interstices
between crystal but will not penetrate the intracrystalline free space.
Crystal framework densities of some typical zeolites, including some which
are not within the purview of this invention, are:
TABLE II
______________________________________
Void Framework
______________________________________
Zeolite Volume Density
Ferrierite 0.28 cc/cc
1.76 g/cc
Mordenite .28 1.7
ZSM-5, 11 .29 1.79
ZSM-12 -- 1.8
ZSM-23 -- 2.0
Dachiardite .32 1.72
L .32 1.61
Clinoptilolite .34 1.71
Laumontite .34 1.77
ZSM-4, Omega .38 1.65
Heulandite .39 1.69
P .41 1.57
Offretite .40 1.55
Levynite .40 1.54
Erionite .35 1.51
Gmelinite .44 1.46
Chabazite .47 1.45
A .5 1.3
Y .48 1.27
______________________________________
Within the above-described group of medium pore size zeolites having a
constraint index of 1-12 is a preferred group of zeolites for the purposes
of the present invention. The preferred members, exemplified by ZSM-22,
ZSM-23, and ZSM-35, are members of a unique class of zeolites. They have
channels described by 10-membered rings of T (=Si or Al) or oxygen atoms,
i.e., they are intermediate pore zeolites, distinct from small pore 8-ring
or large pore 12-ring zeolites. They differ, however, from other
intermediate pore 10-ring zeolites, such as ZSM-5, ZSM-11, ZSM-57 or
stilbite, in having a smaller 10-ring channel. If the crystal structure
(and hence pore system) is known, a convenient measure of the channel
cross-section is given by the product of the dimensions (in angstrom
units) of the two major axes of the pores. These dimensions are listed in
the "Atlas of Zeolite Structure Types" by W. M. Meier and D. H. Olson,
Butterworths, publisher, Second Edition, 1987. The values of this product,
termed the Pore Size Index, are listed in Table A.
TABLE A
______________________________________
Pore Size Index
Largest Axes of Largest
Pore Size
Type Ring Size
Zeolite Channel, A
Index
______________________________________
1 8 Chabazite 3.8 .times. 3.8
14.4
Erionite 3.6 .times. 5.1
18.4
Linde A 4.1 .times. 4.1
16.8
2 10 ZSM-22 4.4 .times. 5.5
24.2
ZSM-23 4.5 .times. 5.2
23.4
ZSM-35 4.2 .times. 5.4
22.7
ALPO-11 3.9 .times. 6.3
24.6
3 10 ZSM-5 5.3 .times. 5.6
29.1
ZSM-11 5.3 .times. 5.4
28.6
Stilbite 4.9 .times. 6.1
29.9
ZSM-57 (10) 5.1 .times. 5.8
29.6
4 12 ZSM-12 5.5 .times. 5.9
32.4
Mordenite 6.5 .times. 7.0
45.5
Beta (C-56) 6.2 .times. 7.7
47.7
Linde-L 7.1 .times. 7.1
50.4
Mazzite (ZSM-4)
7.4 .times. 7.4
54.8
ALPO.sub.4 -5 7.3 .times. 7.3
53.3
______________________________________
It can be seen that small pore, eight-ring zeolites have a Pore Size Index
below about 20, the intermediate pore, 10-ring zeolites of about 20-31,
and large pore, 12-ring zeolites above about 31. It is also apparent, that
the 10-ring zeolites are grouped in two distinct classes; Type 2 with a
Pore Size Index between about 22.7 and 24.6, and more broadly between
about 20 and 26, and Type 3 with a Pore Size Index between 28.6 and 29.9,
or more broadly, between about 28 and 31.
The zeolites which are especially preferred for this invention are those of
Type 2 with a Pore Size Index of 20-26.
The Type 2 zeolites are distinguished from the other types by their
sorption characteristics towards 3-methylpentane. Representative
equilibrium sorption data and experimental conditions are listed in Table
B.
Type 2 zeolites sorb in their intracrystalline voids at least about 10 mg
and no greater than about 40 mg of 3-methylpentane at 90.degree. C., 90
torr 3-methylpentane, per gram dry zeolite in the hydrogen form. In
contrast, Type 3 zeolites sorb greater than 40 mg 3-methylpentane under
the conditions specified.
The equilibrium sorption are obtained most conveniently in a
thermogravimetric balance by passing a stream of inert gas such as helium
containing the hydrocarbon with the indicated partial pressure over the
dried zeolite sample held at 90.degree. C. for a time sufficient to obtain
a constant weight.
Samples containing cations such as sodium or aluminum ions can be converted
to the hydrogen form by well-known methods such as exchange at
temperatures between 25.degree. and 100.degree. C. with dilute mineral
acids, or with hot ammonium chloride solutions followed by calcination.
For mixtures of zeolites with amorphous material or for poorly
crystallized samples, the sorption values apply only to the crystalline
portion.
This method of characterizing the Type 2 zeolites has the advantage that it
can be applied to new zeolites whose crystal structure has not yet been
determined.
TABLE B
______________________________________
Equilibrium Sorption Data of Medium Pore Zeolites
Amount sorbed, mg per g zeolite
Type Zeolite 3-Methylpentane.sup.a)
______________________________________
2 ZSM-22 20
ZSM-23 25
ZSM-35 25
3 ZSM-5 61
ZSM-12 58
ZSM-57 70
MCM-22 79
______________________________________
.sup.a) at 90.degree. C., 90 torr 3methylpentane
ZSM-22 is more particularly described in U.S. Pat. No. 4,556,477, the
entire contents of which are incorporated herein by reference. ZSM-22 and
its preparation in microcrystalline form using ethylpyridinium as
directing agent is described in U.S. Pat. No. 4,481,177 to Valyocsik, the
entire contents of which are incorporated herein by reference. For
purposes of the present invention, ZSM-22 is considered to include its
isotypes, e.g., Theta-1, Gallo-Theta-1, NU-10, ISI-1, and KZ-2.
ZSM-23 is more particularly described in U.S. Pat. No. 4,076,842, the
entire contents of which are incorporated herein by reference. For
purposes of the present invention, ZSM-23 is considered to include its
isotypes, e.g., EU-13, ISI-4, and KZ-1.
ZSM-35 is more particularly described in U.S. Pat. No. 4,016,245, the
entire contents of which are incorporated herein by reference. Isotypes of
ZSM-35 include ferrierite (P. A. Vaughan, Acta Cryst. 21, 983 (1966));
FU-9 (D. Seddon and T. V. Whitam, European Patent B-55,529, 1985); ISI-6
(N. Morimoto, K. Takatsu and M. Sugimoto, U.S. Pat. No. 4,578,259, 1986);
monoclinic ferrierite (R. Gramlich-Meier, V. Gramlich and W. M. Meier, Am.
Mineral. 70, 619 (1985)); NU-23 (T. V. Whittam, European Patent A-103,981,
1984); and Sr-D (R. M. Barrer and D. J. Marshall, J. Chem. Soc. 1964, 2296
(1964)). An example of a piperidine-derived ferrierite is more
particularly described in U.S. Pat. No. 4,343,692, the entire contents of
which are incorporated herein by reference. Other synthetic ferrierite
preparations are described in U.S. Pat. Nos. 3,933,974; 3,966,883;
4,000,248; 4,017,590; and 4,251,499, the entire contents of all being
incorporated herein by reference. Further descriptions of ferrierite are
found in Bibby et al, "Composition and Catalytic Properties of Synthetic
Ferrierite," Journal of Catalysis, 35, pages 256-272 (1974).
The zeolite catalyst used is preferably at least partly in the hydrogen
form, e.g., HZSM-22, HZSM-23, or HZSM-35. Other metals or cations thereof,
e.g., rare earth cations, may also be present. When the zeolites are
prepared in the presence of organic cations, they may be quite inactive
possibly because the intracrystalline free space is occupied by the
organic cations from the forming solution. The zeolite may be activated by
heating in an inert or oxidative atmosphere to remove the organic cations,
e.g., by heating at over 500.degree. C. for 1 hour or more. Other cations,
e.g., metal cations, can be introduced by conventional base exchange or
impregnation techniques.
The zeolite may be incorporated in another material usually referred to as
a matrix or binder. Such matrix materials include synthetic or naturally
occurring substances as well as inorganic materials such as clay, silica
and/or metal oxides. The latter may be either naturally occurring or in
the form of gelatinous precipitates or gels including mixtures of silica
and metal oxides. Naturally occurring clays which can be composited with
the zeolite include those of the montmorillonite and kaolin families,
which families include the subbentonites and the kaolins commonly known as
Dixie, McNamee, Georgia and Florida clays or others in which the main
mineral constituent is halloysite, kaolinite, dickite, nacrite or
anauxite. Such clays can be used in the raw state as originally mined or
initially subjected to calcination, acid treatment or chemical
modification.
In addition to the foregoing materials, the zeolites employed herein may be
composited with a porous matrix material, such as silica, alumina,
zirconia, titania, silica-alumina, silica-magnesia, silica-zirconia,
silica-thoria, silica-beryllia, silica-titania as well as ternary
compositions such as silica-alumina-thoria, silica-alumina-zirconia,
silica-alumina-magnesia and silica-magnesia-zirconia. The matrix can be in
the form of a cogel. A mixture of these components could also be used.
Of all the foregoing materials, silica may be preferred as the matrix
material owing to its relative inertness for catalytic cracking reactions
which are preferably minimized in the instant isomerization processes. The
relative proportions of finely divided zeolite and inorganic oxide gel
matrix vary widely with the zeolite content ranging from about 1 to about
90 percent by weight and more usually in the range of about 30 to about 80
percent by weight of the composite.
The regeneration of spent zeolite catalyst used in the isomerization
reaction is carried out oxidatively or hydrogenatively employing
procedures known in the art. The catalyst of the present invention can be
readily reactivated without significantly reducing selectivity for
iso-olefin by exposing it to hydrogen for a suitable period, e.g.,
overnight.
In order to obtain desired linear olefin skeletal isomerization
activity/selectivity, the zeolite, preferably in the hydrogen form, should
have an alpha value of at least 1, preferably at least 10 when used in the
catalyst of the present invention. Alpha value, or alpha number, of a
zeolite is a measure of zeolite acidic functionality and is more fully
described together with details of its measurement in U.S. Pat. No.
4,016,218, J. Catalysis, 6, pp. 278-287 (1966) and J. Catalysis, 61, pp.
390-396 (1980). The experimental conditions cited in the latter reference
are used for characterizing the catalysts described herein.
The surface acidity of the catalyst can be determined by conversion of
tri-tertiarybutylbenzene (TTBB), a bulky molecule that can only react with
the acid sites on the zeolite crystal surface. Dealkylation of TTBB is a
facile, reproducible method for measuring surface acidity of catalysts.
External surface acidity can be measured exclusive of internal activity
for zeolites with pore diameters up to and including faujasite. As a test
reaction, dealkylation of TTBB occurs at a constant temperature in the
range of from 25.degree. to 300.degree. C. and preferably in the range of
from about 200.degree. to 260.degree. C.
The experimental conditions for the test used herein include a temperature
of 200.degree. C. and atmospheric pressure. The dealkylation of TTBB is
carried out in a glass reactor (18 cm.times.1 cm OD) containing an 8 g
14/30 Vycor.TM. chip preheater followed by 0.1 g catalyst powder mixed
with Vycor.TM. chips. The reactor is heated to 200.degree. C. in 30 cc/g
nitrogen for 30 minutes to remove impurities from the catalyst sample. Ten
g/hr of TTBB dissolved in toluene (7% TTBB) is injected into the reactor.
The feed vaporizes as it passes through the preheater and passes over the
catalyst sample as vapor. After equilibrium is reached the nitrogen is
switched to 20 cc/min hydrogen. The test is then run for 30 minutes with
the reaction products in a cold trap.
The reaction products are analyzed by gas chromatography. The major
dealkylation product is di-t-butylbenzene (DTBB). Further dealkylation to
t-butylbenzene (TBB) and benzene (B) occurs but to a lesser extent.
Conversion of TTBB is calculated on a molar carbon basis. Dealkylation
product weight %s are each multiplied by the appropriate carbon number
ratio to convert to the equivalent amount of TTBB, i.e., DTBB.times.18/14,
TBB.times.18/10 and B.times.18/6. These values are then used in the
following conversion equation where asterisks indicate adjustment to the
equivalents.
##EQU2##
In addition, thermal background experiments using reactors filled with
Vycor.TM. chips only show no TTBB conversion due to Vycor.TM. or other
reactor components.
Limiting surface acidity of the above catalysts is desirable for preventing
undesired reactions on the zeolite surface which are not subject to the
shape-selective constraints imposed upon those reactions occurring within
the zeolite interior. However, reducing the surface acidity will generally
effect a reduction in overall activity of the zeolite. The present
invention relates to the treatment of the zeolite which is contacted with
dicarboxylic acid under conditions resulting in a reduction in surface
acidity (as measured by tri-tertiarybutylbenzene conversion) of at least
20%, preferably at least 40%, more preferably at least 50%. Such reduction
of surface acidity can occur without a significant reduction in overall
activity as measured by alpha test. By significant reduction in overall
activity is meant a reduction in alpha value of not greater than 20%.
The surface acidity of the zeolite can be reduced by dealumination of the
zeolite surface. Performance measures typically improved by dealumination
include product selectivity, product quality and catalyst stability.
Conventional techniques for zeolite dealumination include hydrothermal
treatment, mineral acid treatment with HCl, HNO.sub.3, and H.sub.2
SO.sub.4, and chemical treatment with SiCl.sub.4 or EDTA. The treatments
are limited, in many cases, in the extent of dealumination by the onset of
crystal degradation and loss of sorption capacity. U.S. Pat. No. 4,419,220
to LaPierre et al discloses that dealumination of zeolite Beta via
treatment with HCl solutions is limited to SiO.sub.2 /Al.sub.2 O.sub.3
ratios of about 200 to 300 beyond which significant losses to zeolite
crystallinity are observed.
U.S. Pat. No. 3,442,795 to Kerr et al. describes a process for preparing
highly siliceous zeolite-type materials from crystalline aluminosilicates
by means of a solvolysis, e.g., hydrolysis, followed by a chelation. In
this process, the acid form of a zeolite is subjected to hydrolysis, to
remove aluminum from the aluminosilicate. The aluminum can then be
physically separated from the aluminosilicate by the use of complexing or
chelating agents such as ethylenediaminetetraacetic acid or carboxylic
acid, to form aluminum complexes that are readily removable from the
aluminosilicate. The examples are directed to the use of EDTA to remove
alumina.
EP 0 259 526 B1 discloses the use of dealumination in producing zeolite
ECR-17. The preferred dealumination method involves a combination of steam
treatment and acid leaching, or chemical treatments with silicon halides.
The acid used is preferably a mineral acid, such as HCl, HNO.sub.3 or
H.sub.2 SO.sub.4, but may also be weaker acids such as formic, acetic,
oxalic, tartaric acids and the like.
U.S. Pat. No. 4,388,177 to Bowes et al. discloses the preparation of a
natural ferrierite hydrocracking catalyst by treatment with oxalic acid to
impart catalytic activity for converting slightly branched as well as
straight chain hydrocarbons in hydrodewaxing and naphtha upgrading.
The present invention provides a process for the selective dealumination of
the zeolite used in skeletal isomerization of n-olefins at the zeolite
crystal surface by contacting the zeolite with dicarboxylic acid. The
treatment with dicarboxylic acid is believed to remove aluminum from the
crystal surface of the zeolite.
The invention therefore includes a process for the dealumination of the
zeolite which comprises contacting with dicarboxylic acid for a sufficient
time to effect greater than about 20, 40 or even 50% dealumination of the
crystal surface.
Prior to or following contact with dicarboxylic acid, the zeolite may be
composited with a porous matrix material, such as alumina, silica,
titania, zirconia, silica-alumina, silica-magnesia, silica-zirconia,
silica-thoria, silica-beryllia, silica-titania as well as ternary
compositions, such as silica-alumina-thoria, silica-alumina-zirconia,
silica-alumina-magnesia, and silica-magnesia-zirconia. The matrix may be
in the form of a cogel. The relative proportions of zeolite component and
inorganic oxide gel matrix may vary widely with the zeolite content
ranging from between 1 to 99, more usually 5 to 80, percent by weight of
the composite.
Suitable dicarboxylic acids for use in the process of this invention
include oxalic, malonic, succinic, glutaric, adipic, tartaric, maleic,
phthalic, isophthalic, terephthalic, fumaric or mixtures thereof. Oxalic
acid is preferred. The dicarboxylic acid may be used in solution, such as
an aqueous dicarboxylic acid solution.
Generally, the acid solution has a concentration in the range from about
0.01 to about 4M. Preferably, the acid solution concentration is in the
range from about 1 to about 3M.
The dicarboxylic acid is generally in a volume solution to volume catalyst
ratio of at least about 1:1, preferably at least about 4:1.
Treatment time with the dicarboxylic acid solution is as long as required
to provide the desired reduction in surface acidity. Generally the
treatment time is at least about 10 minutes. Preferably, the treatment
time is at least about 1 hour.
More than one dicarboxylic acid treatment step may be employed in the
process of the present invention for enhanced deactivation of surface
acidity.
The treatment temperature is generally in the range from about 32.degree.
F. to about reflux. Preferably, the treatment temperature is from about
15.degree. C. to 93.degree. C. (60.degree. F. to 200.degree. F.), and more
preferably from 49.degree. C. to 82.degree. C. (120.degree. F. to
180.degree. F.).
The dicarboxylic acid treatment of this invention may also be combined with
other conventional dealumination techniques, such as steaming and chemical
treatment.
The following examples illustrate the process of the present invention.
EXAMPLE 1
Preparation of As-Synethsized ZSM-35
1.18 parts of aluminum sulfate (17.2% Al.sub.2 O.sub.3) were added to a
solution containing 9.42 parts H.sub.2 O and 1.38 parts of 50% NaOH
solution in an autoclave. 0.03 parts of ZSM-35 seeds and 3.20 parts of
Hi-Sil precipitated silica were added with agitation, followed by 1.0 part
of pyrrolidine.
The reaction mixture had the following composition, in mole ratios:
______________________________________
SiO.sub.2 /Al.sub.2 O.sub.3
21.5
OH.sup.- /SiO.sub.2
0.11
H.sub.2 O/Al.sub.2 O.sub.3
13.5
R/Al.sub.2 O.sub.3
6.45
______________________________________
where R=pyrrolidine. The mixture was crystallized at 105.degree. C. for 74
hours with stirring. The ZSM-35 product was filtered, washed with
deionized water, and dried at 120.degree. C.
The chemical composition of the product was, in weight percent:
______________________________________
SiO.sub.2 76.7
Al.sub.2 O.sub.3
6.4
Na 0.84
C 7.26
N 2.03
Ash @ 1000.degree. C.
85.5
______________________________________
with a silica/alumina ratio for the product, in moles, of 20.3/1.
EXAMPLE 2
Preparation of Silica-Bound HZSM-35
A catalyst was prepared by dry mixing 65 parts of the as-synthesized ZSM-35
of Example 1 with 35 parts of precipitated silica, in proportion to give,
after calcination, 65% ZSM-35 / 35% silica in the catalyst. A solution
containing 2% NaOH (based on solids) was added to the mix to create an
extrudable mull, the mix was extruded to 1/16 inch (1.6 mm) diameter and
dried at 120.degree. C. The extrudate was exchanged two times with 1N
NH.sub.4 NO.sub.3 solution at room temperature, rinsed with deionized
water, dried at 120.degree. C. and calcined in nitrogen for 3 hours at
538.degree. C. It was again exchanged with 1N NH.sub.4 NO.sub.3 solution
two times at room temperature, dried at 120.degree. C., and calcined in
air for 9 hours at 538.degree. C.
The resulting catalyst had an alpha activity of 102 and a surface acidity
of 18. Alpha activity was measured using n-hexane, a relatively small
molecule which can access the intracrystalline active sites and external
sites, representing overall catalyst activity. Surface acidity was
measured using tri-tertiarybutylbenzene, a bulky molecule that only reacts
on the zeolite crystal surface. Surface acidity is determined in
accordance with the procedure set out above.
EXAMPLE 3
Oxalic Acid Treated ZSM-35
A catalyst was prepared by treating a portion of the catalyst of Example 2
with 2M oxalic acid at 71.degree. C. for one hour. The treated sample was
washed with water, dried at 150.degree. C., and calcined at 375.degree. C.
for 3 hours. The resulting oxalic acid treated catalyst exhibited an alpha
value (activity) of 92 and a surface acidity of 10. The 10% change in
alpha activity from the untreated to treated catalyst is well within the
accuracy of the alpha test. However, the 45% reduction in surface acidity
represents a significant decrease.
EXAMPLE 4
Preparation of ZSM-23
ZSM-23 was prepared by charging 85.5 parts water to an autoclave followed
by 2.64 parts KOH solution (45% by weight), 1.0 part aluminum sulfate
(17.2% Al.sub.2 O.sub.3) and 0.5 parts ZSM-23 seeds (100% basis). After
mixing thoroughly, 14.5 parts of Ultrasil VN3 precipitated silica
(Nasilco), then 5.1 parts of pyrrolidine were added and mixed thoroughly.
The autoclave was heated to 160.degree. C. with stirring and maintained at
these conditions until crystallization was complete. The product was
identified as ZSM-23 by X-ray diffraction. After flashing the pyrrolidine,
the slurry was cooled, washed, filtered and dried. 65 parts of the dried
ZSM-23 were combined with 35 parts of SiO.sub.2 (Hi-Sil, a product of PPG
Industries Chemical Division, dry mulled and extruded to form 1/16 inch
pellets which were dried at 120.degree. C. The pellets were then calcined
in flowing nitrogen for 2 hours at 538.degree. C. and 3 hours in air at
the same temperature. The cooled catalyst was exchanged with 1 N NH.sub.4
NO.sub.3 (5 cc/g catalyst) at room temperature for one hour then washed
with water. The exchange procedure was repeated and the catalyst dried at
120.degree. C. The exchanged extrudate was then calcined at 538.degree. C.
in flowing air for 3 hours. The resulting catalyst exhibited an alpha
activity of 27 and a surface acidity of 3.
EXAMPLE 5
A sample of ZSM-23 from Example 4 was treated with 2M oxalic acid at
71.degree. C. for one hour. The treated sample was washed with water,
dried at 150.degree. C. for 8 hours, and calcined at 375.degree. C. for 3
hours. The resulting catalyst had an alpha value of 33 and a surface
acidity of 1.5. While the reduction in alpha value is within the accuracy
of the alpha test, the 50% reduction in surface acidity represents a
significant decrease.
EXAMPLE 6
Isomerization of 1-Butene with ZSM-35 at 400.degree. C.
The catalysts of Examples 2 and 3 were sized to 14/24 mesh and used in
butene skeletal isomerization reactions. The approximate experimental
conditions were:
______________________________________
Temperature 400.degree. C.
Pressure 207 kPa
1-Butene WHSV 33 hr.sup.-1 (based on active component)
N.sub.2 /Butene in feed
1 vol/vol
______________________________________
The Figure graphically depicts the respective conversions and isobutene
yields obtained for treated and untreated ZSM-35. Isobutene yield from the
untreated catalyst dropped from more than 40% to less than 30% in less
than 11 days. However, the same drop in yield with the treated catalyst
took over 16 days, representing a 40 to 50% reduction in aging rate.
While the instant invention has been described by specific examples and
embodiments, there is no intent to limit the inventive concept except as
set forth in the following claims.
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